With this set-up I see the Tor process consuming 2% of CPU, about 60MB of RAM (RSS) used 100 - 200 connections active at any given time.
Seconded. It's not much. And irrespective of hardware, seconded also on using current OS, build libs and Tor. Some OS require setting kernel sysctl to enable extra cpu's or cpu features. BIOS too. But that intel-HT is not worth anything.
My ISP currently provides me with ~ 800 kbps in upload ... 15 Mbps in download but I guess it is better to keep it symmetric
It's bytes in/out of the closed circle that is your box/interfaces. Other than encapsulation differences, non-symmetric is not physically possible.
You can set up OS rate limiting to let Tor freely use whatever when you are not using the pipe. But I don't know how to properly let Tor know you are doing that??? (other than telling Tor it's own rate limit should be the entire possible size of your pipe, as would be the case when Tor is allowed by OS to free run during your non usage. Tor would get squeezed otherwise, but that's probably not too bad.)
I have also thought about using the PC of my parents as a bridge
Not a good idea to subject anyone but yourself to the potential issues of being an exit :) So a non-exit or bridge is better.